Positive photoresist containing 2,3,4-trihydroxybenzophenone and 1,2-naphthoquinone-diazide-5-sulfonyl trisester of 1,3,5-trihydroxybenzene

ABSTRACT

Structures of high resolution in the near UV range and of high sharpness of edge and steepness of edge can be obtained by means of positive photoresists containing, in an organic solvent, in each case essentially at least 
     (a) an alkali-soluble resin 
     (b) a 1,2-naphthoquinone-diazide-5-sulfonyl ester of a trihydroxybenzene isomer 
     (c) an aromatic hydroxy compound 
     and also, if appropriate, further customary additives, and in which the result of component (b) is to give an absorption coefficient of at least 0.5 μm -1  for the photobleachable absorption, and component (c) is present in a concentration of 15-30% by weight, relative to the total solids content.

This application is a continuation of application Ser. No. 325,827,filed 3/20/89, abandoned.

The invention relates to positive photoresists with high resolution inthe near UV.

The production of semiconductor devices and integrated circuits inmicroelectronics is carried out virtually exclusively with the use ofphotolithographic structuring processes. In order to prepare microchipshaving specific circuit patterns, the semiconductor substrate material,which is generally silicon wafers, is coated with photoresist andphotoresist relief structures are produced thereon by image-wiseexposure and subsequent development. These structures serve as a maskfor the actual structuring processes on the semiconductor substrate,such as etching, doping or coating with metals or other semiconductor orinsulating materials. In these processes the photoresist masks areremoved again, if necessary. The circuit patterns of the microchips areformed on the substrate by means of a large number of process cycles ofthis type.

In principle, two different types of photoresists are distinguished: Inpositively operating photoresists the exposed areas are dissolved awayby a development process, whereas the unexposed areas remain as a layeron the substrate. In negatively operating photoresists, conversely, theirradiated areas of the layer remain as a relief structure. By virtue oftheir nature, positive photoresists possess a higher image resolutionand are therefore mainly employed in the production of VLSI circuits.

Positive photoresists of the customary type contain, in an organicsolvent, in each case essentially at least one resin soluble in aqueousalkalis and a photosensitive quinone-diazide compound which reduces thesolubility in alkali of the resin. As a result of the action ofradiation on photoresists produced by means of compositions of this typethe solubility in alkali in the exposed areas is increased throughphotoinduced structural transformation of the quinone-diazide compoundinto a carboxylic acid derivative, so that, after development in aqueousalkaline developing baths, positive photoresist relief structures areobtained.

Photosensitive quinone-diazide compounds in use are preferablyesterification products of 1,2-naphthoquinone-2-diazide-5-sulfonic acidor 1,2-naphthoquinone-2-diazide-4-sulfonic acid with low-moleculararomatic hydroxy compounds, in particular hydroxybenzophenones, such as2,3,4-trihydroxybenzophenone and 2,3,4,4'-tetrahydroxybenzophenone andalso trihydroxybenzenes. These naphthoquinone-diazide compounds have abroad absorption in the near to medium UV wavelength range betweenapprox. 300 and 450 nm. Strong emission lines of the mercury vapourlamps customarily employed in the projection equipment, such as, forinstance, the lines at 313 nm, 334 nm, 365 nm, 405 nm, and 436 nm, arelocated within this wavelength range.

Furthermore, positive photoresists can usually also contain modifyingadditives, especially additives for increasing the sensitivity and forcontrolling the development rate and also radiation-absorbing agents anddyes for suppressing reflection and scattering effects which reduceresolution.

The characteristics of a photoresist which are essential forapplicability in industry are sensitivity to radiation, image resolutionand contrast.

A high sensitivity is important in order to ensure short irradiationtimes in manufacturing process cycles and also, for instance, if, byvirtue of the technical characteristics of the equipment, only radiationof a fairly low intensity, for example in the case of monochromaticexposure, can act on the resist.

The image resolution characterizes the degree to which the dimensions ofthe smallest image structures in the original, such as, for instance,lines and interspaces, can be clearly separated and reproduced true todimensions by the photoresist in the form of ridges and trenches. Theproduction of VLSI circuits requires the reproduction of structuraldetails of an order of magnitude of 1 μm or less.

The contrast characterizes the steepness of edges and sharpness of edgesin the photoresist relief structures obtained after development. Theseshould be as sharp-edged as possible and in the ideal case should have90° flanks.

The resolution which can be achieved in the production of photoresiststructures depends primarily on the specific properties of the materialof the photoresist, such as, in particular, the absorption and quantumyield of the radiation initiating the reaction and the developmentbehaviour; its lower limit is determined by the optical quality of theexposure equipment and, in theory, by the wavelength of the radiationused. Lens systems designed for the highest requirements in the case ofmonochromatic exposure must have a high numerical aperture. However, asthe numerical aperture increases, the depth of focus and hence thefocussing tolerance required for use under production conditions, fallsoff considerably. It is not possible to achieve a higher resolution byusing radiation of a lower wavelength, such as, for instance, UVradiation in the far region or X-rays, with the customary positivephotoresists because of their photochemical properties and with thecustomary exposure equipment which operates in the near UV by means ofradiation of the mercury spectrum. Practical limits are thus set toincreasing resolution by means of optical measures.

The positive photoresists which are customary at the present time andbased on an alkali-soluble resin and naphthoquinone-diazide compoundsmake possible resolution down to 0.8-0.9 μm when exposed in the near UVrange using lenses of a numerical aperture of 0.3-0.4. With the demandfor an increasingly higher integration density on microchips there is,therefore, a great need for positive photoresists of higher resolutionwhich can be structured within the near UV range and, in particular, canbe processed in the wafer steppers which are employed to a large extentin mass production and which are mainly operated on a monochromaticbasis using the radiation of the mercury G line (436 nm). For thesub-micrometer range an improvement in resolution by 0.1-0.3 μm comparedwith the state of the art ranks as considerable in this respect. It was,therefore, required to improve positive photoresists according to thestate of the art in such a way that they exhibit a markedly increasedresolution in the near UV range and also provide structures of greatsharpness of edge and steepness of edge in the region of maximumresolution.

It has now been found, surprisingly, that positive photoresistsessentially containing, in an organic solvent, in each case at least

(a) an alkali-soluble resin,

(b) a 1,2-naphthoquinone-diazide-5-sulfonyl ester of a trihydroxybenzeneisomer,

(c) an aromatic hydroxy compound

and, if appropriate, further customary additives exhibit a considerablyimproved resolution when exposed to radiation in the near UV range, ifthe result of component (b) in them is to give an absorption coefficientof at least 0.5 μm⁻¹ for the photobleachable absorption and if component(c) is present in a concentration of 15-30% by weight, relative to thetotal solids content.

Positive photoresists having a composition of this type make it possibleto reproduce structural details down to at least 0.6 μm with sharp edgesand with flanks of virtually 90° without problems when exposed to the Gline at a numerical aperture of 0.35.

Admittedly positive photoresists containing the components (a), (b) and(c) indicated are already known from German Patent Application P3,724,791; the positive photoresists described therein had, however, adifferent quantitative composition and accordingly did not exhibit thedesired high resolution either. Nor can the desired high resolution beachieved even with photoresists of a similar composition in respect ofabsorption coefficient and content of aromatic hydroxy compound, butcontaining naphthoquinone-diazide-sulfonyl esters ofhydroxybenzophenones, as described, for instance, in U.S. Pat. No.4,626,492.

The invention therefore relates to positive photoresists containing, inan organic solvent, in each case essentially at least

(a) an alkali-soluble resin

(b) a 1,2-naphthoquinone-diazide-5-sulfonyl ester of a trihydroxybenzeneisomer,

(c) an aromatic hydroxy compound

and, if appropriate, further customary additives, wherein the result ofcomponent (b) is to give an absorption coefficient of at least 0.5 μm⁻¹for the photobleachable absorption and component (c) is present in aconcentration 15-30% by weight, relative to the total solids content.

The invention relates in particular to positive photoresists of thistype in which the component (b) is the1,2-naphthoquinone-diazide-5-sulfonyl trisester of1,3,5-trihydroxybenzene and the component (c) is2,3,4-trihydroxybenzophenone.

The invention also relates to the use of positive photoresists of thistype as photoresists of high resolution in the near UV range.

The invention relates, additionally, to a process for the production ofphotoresist relief structures having a resolution down to at least 0.6μm by coating a semiconductor substrate with a positive photoresist,exposing the coating imagewise to radiation from the near UV wavelengthrange and developing with an aqueuous alkaline developer, using apositive photoresist as characterized above.

As is familiar to those skilled in the art, one parameter by means ofwhich the image-forming properties of a positive photoresist can becharacterized very well is the absorption coefficient of thephotobleachable absorption, the A-value, for a predetermined wavelengthof irradiation (in this regard see, for example: F. H. Dill, W. P.Hornberger, P. S. Hauge, J. M. Shaw, IEE Trans. Electr. Dev. ED-22, 445(1975)). The A-value contains the molar extinction specific to thecompound and the concentration of the photoreactive component of thephotoresist. This absorption falls off during the irradiation of thephotoresist because of the photo-induced transformation. In addition, apart is also played by the absorption coefficient of thenon-photobleachable absorption, the B-value. This contribution to thetotal absorption at the wavelength of the irridation originates from thephotolysis product of the photoreactive component(s) and from the other,non-photoreactive constituents of the photoresist, for example addeddyes.

It is known that better results in respect of sharpness of edges andsteepness of flanks can be achieved with positive photoresists, and alsothat they are less critical in regard to the tolerance of focussing, ifthey have a high A-value. Photoreactive components having a high contentof radiation-sensitive groups in the molecule, for examplenaphthoquinone-diazide-sulfonyl trisesters and tetraesters of aromatichydroxy compounds naturally have a high molar extinction for thephotobleachable absorption and therefore require a lower totalconcentration in the photoresist in order to adjust the A-value to aspecific figure than, for example, the monoesters or bisesters. Itshould, therefore, also be possible to achieve a higher resolution ofstructures with a theoretically consequent higher differentialsolubility by means of a high local concentration of photoreactivecomponent. It has been found, however, that, in the presence of highconcentrations of customary naphthoquinone-diazide-sulfonyl esters oftrihydroxybenzophenones or tetrahydroxybenzophenones, which are normallynot single-substance products, but mixtures of the complete and partialesters, no structures of the desired resolution with high sharpness ofedges and steepness of edges can be obtained because of the increaseddeveloper solubility of this resist. On the other hand, photoresistscontaining a high concentration of the naphthoquinone-diazide-sulfonyltrisesters of the trihydroxybenzenes, which can be obtained assingle-substance products, prove to be virtually no longer capable ofdevelopment. In accordance with the invention it was only possible toobtain structures having a resolution down to at least 0.6 μm with ahigh sharpness of edges and steepness of edges as a result of addingaromatic hydroxy compounds which increase the rate of developing in aconcentration of 15-30% by weight, relative to the total solids contentof the photoresist and if the A-value is adjusted to at least 0.5 μm⁻¹by means of naphthoquinone-diazide-sulfonyl esters of trihydroxybenzeneisomers.

The positive photoresists according to the invention contain, in anorganic solvent, as the resin component an alkali-soluble resin, as thephotosensitive component a 1,2-naphthoquinone-diazide-5-sulfonyl esterof a trihydroxybenzene isomer in a content which results in anabsorption coefficient of at least 0.5 μm⁻¹ for the photobleachableabsorption, and an aromatic hydroxy compound which increases the rate ofdeveloping in a concentration of 15-30% by weight, relative to the totalsolids content. If appropriate, they can also contain further customaryadditives by means of which it is possible, if appropriate, to adjustthe properties of the photoresist to specific requirements of the enduse.

The resin component is, in principle, any alkali-soluble resin which iscustomary in the technology of photoresists, for example novolak resinswhich are obtained by subjecting phenol or phenolic compounds to acondensation reaction with aldehydes. Cresol-formaldehyde resins in thepreparation of which o-, m- or p-cresol or mixtures of these isomers areemployed in any desired or predetermined ratios are preferred. Thepreparation of such resins and their use in positive photoresists isdescribed, for example, in U.S. Pat. No. 4,377,631. In addition, otheralkali-soluble resins such as are frequently used in positivephotoresists are also suitable. These include, for instance,poly-(vinylphenols) and polyglutarimides, copolymers formed from styreneand α-methylstyrene with maleimide and copolymers formed fromN-(p-hydroxyphenyl)-maleimide and olefins. It is also possible to usesilylated derivatives of alkali-soluble polymers having a fairly highresistance to plasma etching. The resin component forms a proportion ofabout 40-70% by weight, relative to the total solids content, in thepositive photoresists according to the invention.

The photosensitive components employed in the positive photoresistsaccording to the invention are 1,2-naphthoquinone-diazide-5-sulfonylesters of trihydroxybenzene isomers. They can be the trisesters of1,2,3-, 1,2,4- and 1,3,5-trihydroxybenzene. These compounds are knownand can be obtained as pure, complete esters in a simple manner byesterifying the corresponding trihydroxybenzene isomers with1,2-naphthoquinonediazide-5-sulfonyl chloride. The isomeric forms ofthese trisesters are normally employed in a pure state, but can also beused as a mixture with one another. Their use in the positivephotoresists according to the invention is effected in a proportion suchthat the result is an absorption coefficient of at least 0.5 μm⁻¹ forthe photobleachable absorption. The A-value to be obtained is preferablywithin the range between 0.55 and 0.75 μm⁻¹ at a wavelength ofirradiation of 436 nm. This is the case, depending on the slightlyvarying molar extinctions of these isomers, above a content of about 17%by weight, relative to the total solids content. The1,2-naphthoquinone-diazide-5-sulfonyl trisester of1,3,5-trihydroxybenzene is the preferred radiation-sensitive component.This ester is preferably employed in a concentration of 17-30% byweight, relative to the total solids content.

The positive photoresists according to the invention also contain, inorder to increase the rate of development, an aromatic hydroxy compoundin a proportion of 15-30% by weight, relative to the total solidscontent.

These are principally compounds of the formula I ##STR1## in which X isa single bond, --O--, --S--, SO₂, CO or CR₅ R₆, R¹, R², R³ and R⁴ areeach H, halogen, C₁ -C₄ alkyl, C₁ -C₄ alkoxy or OH and R⁵ and R⁶ areeach H, C₁ -C₄ alkyl or C₁ -C₁₄ perfluoroalkyl.

The compounds of the formula I are bisphenyl compounds in which eachphenyl ring carries at least one hydroxyl group and in which the twophenyl rings are linked via a bridge X which can be a single bond, anoxygen or sulfur atom, a sulfonyl or carbonyl group or a methylene groupwhich is unsubstituted or substituted by C₁ -C₄ alkyl or C₁ -C₄perfluoroalkyl. Insofar as they are not hydrogen, the radicals R¹, R²,R³ and R⁴ can be halogen, such as fluorine, chlorine or bromine, C₁ -C₄alkyl, such as methyl, ethyl, n-propyl, isopropyl or n-, iso- ort-butyl, C₁ -C₄ -alkoxy, such as methoxy, ethoxy, n-propoxy, iso-propxyor n-, iso- or t-butoxy, and OH.

The compounds of the formula I preferably contain two, three or four OHgroups and in other respects no further substituents. The OH groupspreferably occupy the 4,4'-, 2,2'-, 2,3,4- or 2,2',4,4'-positions.Preferred additives are, for instance, the compounds2,3,4-trihydroxybenzophenone, 4,4'-dihydroxybiphenyl,2,2'-dihydroxybiphenyl, 4,4'-dihydroxybenzophenone,bis-(4-hydroxyphenyl) ether, bis-(4-hydroxyphenyl) sulfide,bis-(2,4-dihydroxyphenyl) sulfide, bis-(4-hydroxyphenyl) sulfone or2,2-bis-(4-hydroxyphenyl)-propane. 2,3,4-Trihydroxybenzophenone,bis-(4-hydroxyphenyl) sulfone, 4,4'-dihydroxybenzophenone andbis-(2,4-dihydroxyphenyl) sulfide are particularly preferred and ofthese 2,3,4-trihydroxybenzophenone is particularly preferred.

The positive photoresists according to the invention typically contain50 to 65% by weight of an alkali-soluble resin, 18 to 25% by weight ofthe 1,2-naphthoquinone-diazide-5-sulfonyl trisester of1,3,5-trihydroxybenzene and 17 to 25% by weight of2,3,4-trihydroxybenzophenone, in each case relative to the total solidscontent.

Solvents suitable for the preparation of the photoresist solution are,in principle, any solvents in which the solid photoresist constituents,such as the alkali-soluble resin, the quinone-diazide compound, thearomatic hydroxy compound and, if appropriate, further additives areadequately soluble and which do not react irreversibly with theseconstituents. Examples of solvents suitable for this purpose arealiphatic ketones, such as methyl ethyl ketone, cyclopentanone orcyclohexanone, aliphatic esters, such as butyl acetate, ethers, such asanisole or tetrahydrofuran, alcohols such as n-propanol or isopropanol,mono ethers or bis ethers and mixed ether-ester derivatives of glycolcompounds such as ethylene glycol, diethylene glycol or propyleneglycol, also monooxycarboxylic acid esters, such as, for instance, ethyllactate or ethyl 2-ethoxypropionate, lactones, such as γ-butyrolactone,or cyclic amides, such as 2-methylpyrrolidone. Aliphatic and aromatichydrocarbons, such as n-hexane and xylene, are also used as solvents.Mixtures of the said solvents are also frequently used. Photoresistsolvents commonly contain ethoxyethyl acetate, methoxypropyl acetate orethylene glycol dimethyl ether. The solvent is usually present in aproportion of 40-90% by weight of the total photoresist solution.

The further customary additives which can also be present in thephotoresists according to the invention include, for instance,substances which absorb scattered radiation, dyes, flow control agents,plasticizers, adhesion promoters, further film-forming resins,surfactants and stabilizers. Additives in these categories areadequately known to those skilled in the art and are widely described inthe relevant specialist literature. The proportion of additives of thistype, taken together, hardly exceeds 25% by weight, relative to thetotal solids content of the photoresist solution.

The positive photoresists according to the invention are formulated in amanner known per se by mixing or dissolving the components in thesolvent or solvent mixture. After dissolving the constituents in thesolvent, the resultant photoresist solution is filtered through amembrane filter of pore width 0.1-1 μm, depending on the extent to whichit must be free from particles. Customarily, the total solids content ofthe photoresist is adjusted to suit the desired layer thickness andcoating method.

The application is effected in a manner known per se and using theprocess equipment customary for this purpose, by coating a substratewith the photoresist solution, drying the layer at an elevatedtemperature, exposing the layer imagewise to radiation of a wavelengthrange in which the layer is sensitive and developing with an aqueousalkaline developer.

The substrates used are mainly semiconductor wafers, for example siliconwafers, which, if desired, can be coated with a layer of silicondioxide, silicon nitride or aluminium. Other materials customary in theproduction of miniaturized circuits, such as germanium, gallium arsenideor ceramics, if appropriate with a noble metal coating, are alsosuitable.

Coating is normally effected by dipping, spraying, rolling orwhirler-coating. In the latter method of coating, which is the mostfrequently used, the resulting layer thickness depends on the viscosityof the photoresist solution, the solids content and the whirler-coaterspeed. So-called whirler curves are determined for each particularphotoresist, and from these the resist layer thicknesses can bedetermined as a function of viscosity and whirler speed of rotation. Thelayer thicknesses for positive photoresists are typically within therange from 0.5 to 4 μm, especially 0.6-2.0 μm.

After the photoresist has been applied to the substrate it is normallysubjected to preliminary drying at temperatures between 70° C. and 130°C. It is possible to use ovens or hotplates for this purpose. The dryingtime in ovens is within a range of about 15-45 minutes and on hotplatesis within a range of about 0.5-4 minutes. It is preferable to dry forabout 1 minute at about 100° C. on a hotplate.

The positive photoresists according to the invention are preferablyexposed to monochromatic light at 436 nm, when their excellentresolution properties become evident in a particularly advantageousmanner.

The substrates coated with the photoresist and exposed are finallydeveloped with an aqueous alkaline developer solution, for example bydipping or spraying, until the resist in the exposed areas has beencompleted dissolved away. Various developer formulations belonging tothe class of photoresist developers either containing metal ions of freefrom metal ions can be used. Developers containing metal ions areaqueous solutions of sodium hydroxide or potassium hydroxide which canalso contain pH-regulating and buffering substances, such as phosphatesor silicates, and also surfactants and stabilizers. Developers free frommetal ions contain, instead of alkaline metal compounds, organic bases,for example tetramethylammonium hydroxide or choline. The developmenttimes depend on the exposure energy, the strength of the developer, thetype of development, the pre-drying temperature and the temperature ofthe developer. Development times of about 1 minute are typical in thecase of dip development. Development is usually stopped by immersion inor spraying with deionized water. Development is frequently followed byan after-drying at about 100° C. to 180° C.

Relief structures produced by means of the photoresists according to theinvention exhibit an excellent image resolution down to at least 0.6 μmwith a high contrast and high steepness of edge and sharpness of edge.The loss of layer thickness in the unexposed areas is minimal. In thesubsequent process sequences in the production of integratedsemiconductor circuits, such as etching with acid or in plasma, dopingor coating, the photoresists exhibit excellent properties and ensureeffective protection of the areas of the substrate which are covered bythe photoresist relief structures.

EXAMPLES

A. Photoresist formulations

Unless otherwise specified, photoresist formulations were prepared fromthe components

(a) an m-kresol Novolak resin of average molecular weight M_(w) =9000

(b) the 1,2-naphthoquinone-diazide-5-sulfonyl trisester of1,3,5-trihydroxybenzene and

(c) 2,3,4-trihydroxybenzophenone

in the percentages indicated under C, relative to the total solidscontent, in the form of 35% solutions in diethylene glycol dimethylether, and these solutions were filtered through filters having a porewidth of 0.2 μm.

B. Test methods

The photoresist formulations were whirler-coated onto surface-oxidizedsilicon wafers of diameter 4", the whirler speed of rotation being sochosen that, after drying on a hotplate for 1 minute at 100° C., a layerthickness of 1.5 μm resulted in each case. The layer was then exposedthrough a resolution test mask to monochromatic light of 436 nm using alens system of numerical aperture NA=0.35 and was then developed byimmersion in 1.62% aqueous tetramethylammonium hydroxide solution at 20°C. for 60 seconds.

The resulting resist structures were inspected under a scanning electronmicroscope in regard to resolution and sharpness of edge.

C. Results

The table below shows the resulting absorption coefficients for thephotobleachable absorption (A-value) and the maximum image resolutiontogether with characterization of the edge profiles for photoresistformulations according to the invention (Examples 1 and 2) and alsocomparison formulations (Examples 3 to 6), at varying percentages of thecomponents (a), (b) and (c).

    ______________________________________                                              Components                                                              Ex-   (% by weight)                                                           ample (a)    (b)    (c)  A-value                                                                              Resolution/edge profile                       ______________________________________                                        1     60.0   18.0   22.0 0.54   0.6 μm/vertical edges                      2     62.4   18.6   19.0 0.56   0.6 μm/vertical edges                      3     72.0   10.0   18.0 0.30   0.8 μm/no vertical edges;                                                      not dimensionally                                                             stable                                    4     70.2   14.4   15.4 0.41   0.9 μm/no vertical edges                   5     67.0   21.0   12.0 0.65   0.9 μm/vertical edges                      6     68.0   20.4*  11.6 0.65   0.9 μm/no vertical edges                   ______________________________________                                         *Esterification product of 1 mol of 2,3,4,4tetrahydroxybenzophenone with      mol of 1,2naphthoquinone-diazide-5-sulfonyl chloride.                    

It is found that structures of a resolution down to 0.6 μm and having avertical and sharp edge profile can only be obtained with the positivephotoresist formulations according to the invention.

What is claimed is:
 1. A positive photoresist comprising, in an organicsolvent, (a) an alkali-soluble resin, (b)1,2-naphthoquinone-diazide-5-sulfonyl trisester of1,3,5-trihydroxybenzene, and (c) 2,3,4-trihydroxybenzophenone; component(b) and component (c) each being present in a concentration of 17-30% byweight, relative to the total solids content.
 2. A positive photoresistaccording to claim 1, which contains(a) 50 to 56% by weight of analkali-soluble resin, (b) 18 to 25% by weight of the1,2-naphthoquinone-diazide-5-sulfonyl trisester of1,3,5-trihydroxybenzene and (c) 17 to 25% by weight of2,3,4-trihydroxybenzophenone, in each case relative to the total solidscontent.